The invention relates to a colour display device comprising a cathode ray tube, a means for generating at least one electron beam, a display screen and a deflection unit for generating deflection fields for deflecting electron beam(s) across the display screen.
The invention also relates to a deflection unit for a cathode ray tube. The earth""s magnetic field or other ambient magnetic fields cause a distortion of the image displayed. The most prominent of these effects is commonly referred to as image rotation or frame rotation.
It has been found that the earth""s magnetic field or indeed any other ambient magnetic field does not only cause a frame rotation but also other errors. A serious problem is given by the fact that apart from an image rotation also a shift of the picture in the horizontal direction as well as a trapezium distortion of the displayed image is caused by such fields. The trapezium distortion is one particular annoying. Customers want to use colour display devices for world wide application. Because of ever increasing demand on (reduction of) geometry distortions in the displayed image, rejects on geometry distortion are ever more increasing.
It is an object of the invention to provide a display device having an improved quality of the displayed image.
To this end, the display device in accordance with the invention is characterised in that the display device comprises a coil circumferentially arranged around the display device and the display device comprises means for directing an ac current through the coil, said accurrent having the same frequency as the vertical deflection current for generating a magnetic field for compensation of a trapezium shaped distortion of the displayed image.
The inventors have realised that the cause of the problem is the amplitude and direction of the vertical field component of the ambient field, usually the earth magnetic field, besides a shift in horizontal direction, it gives a trapezium distortion of the displayed image.
Because this vertical component is different in the North- and South hemisphere a difference in trapezium distortion occurs. In a 17xe2x80x3 diagonal display device the difference is typical 1.6 mm, a clearly visible effect. Thus a display tube optimised for the Northern hemisphere will show clearly visible trapezium distortion when used in the Southern hemisphere. This effect leads ever more, given the ever more increasing demands on the image quality to rejection of the device. Several possibilities for solving this problem have been considered by the inventors:
Optimise the deflection unit especially for the Southern hemisphere. This will, however effect convergence quality and lead to two different types of deflection units
Use a magnetic 2-pole on the neck of the tube to compensate the ambient magnetic field. This would however seriously effect landing purity and require further adjustments.
These solutions therefore are far from simple solutions and cause other serious problems.
The display device in accordance with the invention provides a simple solution for the problem, which does not introduce substantial additional errors, which would then have to be compensated. It is remarked that the invention is aimed at providing a solution to a problem of trapezium distortion, said problem most commonly being caused by the earth""s magnetic field or other ambient magnetic fields. These are however, not the only possible causes for the occurrence of a trapezium distortion. The invention in its broadest scope can be used whatsoever the cause of the trapezium distortion might be, although certain preferred embodiments as described below are directed to solving problems specifically caused by the earth""s magnetic field.
A coil which is circumferentially arranged is, when driven by an ac current of the proper frequency, capable of compensating the negative effect of the earth""s magnetic field (or any other ambient magnetic field) of trapezium distortion. Supplying the coil with a current having the same frequency as the vertical deflection current enables a field of the right frequency to be formed. It is surprising that although the trapezoidal distortion is usually formed by a static magnetic field component, the compensation is performed by generating an ac magnetic field. The field generated by the circumferentially arranged coil is of a relatively simple shape, and does not or only to a small degree introduce (higher order) field components having detrimental side effects on other image properties. Preferably, the compensation coil is circumferentially arranged around the display device at, near or beyond the end of the deflection unit facing the display screen. This position and arrangement of the coil (namely at a position where the electron beams have already undergone at least a substantial deflection) will cause the coil to generate a compensating field which will counter-effect the trapezium distortion most effectively but has negligible effects on other image properties, such as landing or convergence.
The coil could be approximately circular or show slight deviations from a circular shape (for instance ellipsoidal or rectangular with rounded comers).
The coil is preferably mounted on the deflection unit to simplify the design. The coil could however also be a stand-alone coil on the cone of the tube.
The coil may be housed in a housing to provide protection and increase ease of installment.
In a first embodiment of the invention the display device is provided with means to supply also a dc current to the coil.
In this embodiment the coil generates both a dc field, by which rotation of the image may be (partly) counteracted, as well as an ac field by which a trapezium distortion of the image may be counteracted.
Such an embodiment may comprise a single coil for both rotation and trapezium distortion compensation, or a coil comprising sub-coils in series as far as dc current is concerned, a first sub-coil being supplied in operation with a dc and an ac current, a further sub-coil with an dc current. Use of a single coil has the advantage that only one coil is to be used, use of sub-coils has the advantage that the sub-coils and more in particular the current through the sub-coils and can be optimised to reduce power consumption.
Preferably the coil is arranged in series with the vertical deflection coils. This provides a simple design. The added impedance to the vertical deflection coils is neglectable small compared to that of the vertical coils.
Preferably the coil comprises 5-20 turns, when in series with the deflection unit.
The means for supplying dc current through the coil may be provided by the manufacturer so as to counteract a known effect (for instance when it is known that the device will be used in a particular region or country in the southern hemisphere) or it can be supplied with means for setting the dc current by the user (or seller of the device). Such means for setting may be provided with indication relating to the countries or other indications to help setting the dc current.